This invention relates generally to improvements in line protectors of the type which protect telecommunications equipment from overvoltage or surge voltage conditions, and more particularly to a line protector which uses a solid state device as an overvoltage arrester.
Line or station protectors are used to provide telephone equipment with protection against voltage surges or overvoltages on the lines to which they are connected. Such protectors generally employ a surge arrester for each of the two wires, tip (T) and ring (R), in the telephone line to be protected.
In the recent past, an air-gap arrester has been used consisting of a pair of carbon electrodes which are held in a spaced-apart, facing condition across a small air gap. Upon the occurrence of an overvoltage or surge voltage on the protected line, the voltage causes arcing across the air gap and consequent grounding of the overvoltage condition. In this regard, one of the carbon electrodes is in circuit with ground and the other is in circuit with the line to be protected. One example of such an arrester (often referred to as a carbon arrester) is illustrated in U.S. Pat. No. 3,743,888 which is commonly assigned with the present invention.
Generally speaking, with carbon electrodes, a first electrode is generally a short, cylindrical or disc-shaped member which is of a similar outer diameter to the inner diameter of the cup-shaped housing member and is housed against the bottom or end wall thereof. The other electrode is a generally elongate, cylindrical rod-shaped member of lesser diameter which is, in turn, held within a generally tubular ceramic sleeve. The ceramic sleeve is generally of similar outer diameter to the first electrode and is placed within the housing to abut the outwardly facing surface of the first electrode. The sleeve holds the second electrode spaced apart from t-he first electrode to define an arc-gap of a desired length therebetween. Preferably the bottom of the cup and hence the first electrode is in electrically conductive contact with ground while the second rod-shaped electrode projects outwardly toward an open end of the cup-shaped housing to be placed in electrical circuit with the line to be protected.
A second or alternative type of surge arrester which has been used in station protectors in the recent past is a gas tube type of surge arrester. One example of a station protector which uses a gas tube type of overvoltage arrester is shown in U.S. Pat. No. 4,241,374, which patent is also commonly assigned with the present invention. As shown in the '374 patent a gas tube arrester assembly is placed inside of a conductive cap which is capable of being threaded into a well of a station protector block which holds multiple identical arresters. The gas tube member of the assembly is housed within a conductive cup or cup-like housing. The protector block is constructed for establishing a ground connection with the threaded conductive cap of the assembly. The gas tube is generally cylindrical in form having electrodes at its opposite axial ends. The electrode of the gas tube that is adjacent to the open end of the cup is in contact with a metallic contactor element which, in turn, is in contact with the line circuit to be protected. The other electrode of the gas tube is in contact with the cup-like housing which is in an electrically conductive circuit with the conductive cap and hence with ground. Preferably the assembly further includes an O-ring which fits over the gas tube and holds the same in place within the cup-like housing.
More recently, solid state devices have become available which are usable as overvoltage arresters in place of either gas tubes or carbon arc arresters to protect telecommunications equipment. Heretofore, these solid state devices were not available with the voltage ratings or power handling capabilities necessary for such use. However, with the availability of such solid state devices for this use, there has been increasing interest in replacing older gas tube or carbon elements with the newer solid state devices.
However, with literally millions of installations already in place in the field, it is further desirable to provide protective units which employ solid state overvoltage arresters and which can be used to replace existing line protector modules on a one-for-one or retrofit basis, interfitting with existing protector blocks and similar equipment. This avoids the much greater expense of physically removing and replacing complete installations. That is, protector elements in modular form should preferably be capable of being merely physically plugged into existing protector blocks or panels in existing installations. Hence it is desired to provide protector modules which are physically and electrically identical with existing protector modules, but which substitute solid state device type overvoltage arresters for the currently used carbon or gas tube type arresters.
The solid state arresters are believed to offer a number of advantages over either the carbon or gas tube type of arrestors currently in use. Such solid state arresters generally provide noise-free transmission, and have greatly reduced maintenance requirements and longer service life.
One example of such a line protector employing a solid state arrester is shown in U.S. Pat. No. 4,958,253, which is also commonly assigned with the present invention. This line protector preferably utilizes a thin, disc-like shape solid state of arrester element, which is generally of type shown in U.S. Pat. No. 4,939,619. The line protector shown and described in the '253 patent uses a solid conductive pedestal between one electrode of the arrester and the closed end of a cup or cup-like housing which contains the arrester. The closed end of the cup is in contact with the ground pin of a five-pin type of protector module.
It is desirable then to have a solid state surge arrester assembly which can be used in station protectors as a direct replacement for a gas tube or carbon type of surge arrester of the type referred to above. If this solid state assembly is a direct replacement for the presently used arrester, it can be retrofitted in existing station protector blocks or panels.